Re: [PATCH] static_call,x86: Robustify trampoline patching

Peter Zijlstra
2021-11-02T18:20:55Z

On Tue, Nov 02, 2021 at 10:35:30AM -0700, Kees Cook wrote:
> On Sat, Oct 30, 2021 at 10:16:31AM +0200, Peter Zijlstra wrote:
> > foo.cfi:
> > endbr
> > xorl $0xdeadbeef, %r10d
> > jz foo
> > ud2
> > nop # make it an even 16 bytes
> > foo:
> > # actual function text
> >
> >
> > Then have the address of foo, be the address of foo, like any normal
> > sane person would expect. Have direct calls to foo, go to foo, again, as
> > expected.
> >
> > When doing an indirect call (to r11, as clang does), then, and only
> > then, do:
> >
> > movl $0xdeadbeef, %r10d
> > subq $0x10, %r11
> > call *%r11
> >
> > # if the r11 lives, add:
> > addq $0x10, %r11
> >
> >
> > Then only when caller and callee agree 0xdeadbeef is the password, does
> > the indirect call go through.
> >
> > Why isn't this a suitable CFI scheme even without IBT?
>
> The trouble is that the callee is doing the verification. There's no
> protection against calling into a callee that doesn't perform a check
> (e.g. BPF JIT, or otherwise constructed executable memory, etc). The
> caller needs to do the verification that what they're calling into is
> safe before it makes the call.

Right, Ard said the same, see new crackpot scheme here: